Reworked CompositeConfiguration regarding thread-safety.

The methods for accessing the list of child configurations and the in-memory
configuration now call the Synchronizer correctly. Updated Javadoc regarding
thread-safety.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1483061 13f79535-47bb-0310-9956-ffa450edef68